1. Field
Aspects of the present invention relate to an apparatus and method for recording data on a holographic storage medium.
2. Description of the Related Art
Optical holography stores data using the volume of a recording medium, as opposed to the surface of the recording medium. During a recording mode, a signal beam interferes with a reference beam on the recording medium to produce an interference grating called a data page. In a multiplexing scheme, the optical characteristic of the reference beam is changed while a plurality of interference gratings are superposed. During a reproduction mode, a reference beam is incident on the recording medium under the same condition as for recording data to produce diffracted light representing the stored data page. The diffracted light is detected by a detector array that extracts data bits stored from a measured intensity pattern. The data page contains a number of data bits or pixels. Accordingly, a data storage capacity of the recording medium can be increased by superposing a plurality of data pages in the same volume.
A hologram is recorded using a reference beam and a signal beam containing data. FIGS. 1A and 1B are diagrams to explain recording and reproduction modes in optical holography. Referring to FIG. 1A, during a recording mode, a reference beam R and a signal beam S interfere with each other to produce an interference pattern and send the interference pattern to a medium. Referring to FIG. 1B, during a reproduction mode, the reference beam R is emitted to a hologram recorded on the medium to cause diffraction from the recorded hologram such that the signal beam S is reproduced and output. If the reference beam used for reproducing is different from the reference beam used for recording, the intensity and direction of the reproduced signal beam are different from those of the original signal beam recorded on the medium. In general, when the difference increases, the light intensity decreases in the form of a sinc function.
FIG. 2 is a diagram illustrating the angle of a signal beam according to each region when data is recorded on a holographic storage medium. Referring to FIG. 2, a signal beam and a reference beam are incident on the holographic storage medium. The signal beam is modulated by a light modulator (e.g., spatial light modulator (SLM)), and concentrated on a holographic storage medium in the form of a page. The SLM is a membrane device such that the angle of the signal beam incident on the holographic storage medium varies according to each region of the SLM. When the region of the SLM is divided into regions a, b, c, and d in a scanning direction, as shown in FIG. 2, the incident angle and selectivity of the signal beam at each region are shown in Table 1.
TABLE 1Region aRegion bRegion cRegion dIncident angle35.8628.6221.3814.14of signal beam(°)Selectivity (°)0.110.120.140.16
The angle of the signal beam incident on the holographic storage medium, which is the angle of the signal beam to a normal line of the holographic storage medium, is 35.86° at the region a, 28.62° at the region b, 21.38° at the region c, and 14.14° at the region d of the SLM. Each of the regions has an angle deviation of about 7.24° since the signal beam passes through an objective lens having a numerical aperture (NA), which concentrates the signal beam, before entering the holographic storage medium. In general, incident light is refracted outwardly unless the light is incident within a certain angle from a central axis. The NA of the objective lens is the sine of the maximum angle of incident light that is totally reflected and transmitted in the lens without being refracted outwardly. The NA is found from the calculation of the selectivity of the signal beam at each region that a smaller angle results in higher selectivity. That is, the selectivity of the signal beam varies according to each region of the SLM, and a higher selectivity is preferred to prevent crosstalk. However, as selectivity increases, an angular separation of each hologram increases, thereby making it difficult to achieve high density multiplexing of the holographic storage medium.
FIG. 3 is a diagram illustrating the region of a page, which is a signal beam modulated by an SLM, divided into regions A, B, C, and D in a scanning direction of a reference beam. FIG. 4 is a graph illustrating the selectivity of the signal beam according to each region of the page of FIG. 3. The selectivity varies according to each region, like in Table 1 of FIG. 2. Since the maximum selectivity should be selected, it is difficult to achieve high density data recording.